I. Field of the Invention
This invention relates to a process and apparatus for the production of shaped continuous metallic filaments directly from the melt by casting a stream of molten metal into a quenching groove on the inside of a revolving annular chill roll, the groove being flanked by a smooth inclined surface of a material which has low thermal conductivity, is relatively non-quenching and which is not wetted by the molten metal.
II. Description of the Prior Art
Research in recent years has been directed toward the development of methods of filament formation which avoid the restrections of die drawing or rolling. One of the approaches under investigation involves free casting or direct melt spinning and concerns the formation of a free jet of molten fluid and the transformation of the jet to the solid state. This procedure may be readily employed to form filaments of polymeric materials and oxide glasses, i.e. materials having very high viscosities and low surface tension in the liquid state. In contrast, however, metals have relatively inviscid melts of high surface free energy. A cylindrical jet of such a material is inherently unstable. Its surface becomes increasingly perturbed as it issues from the nozzle until at some distance the jet breaks up into droplets. Accordingly, a process, if it is to be capable of producing continuous metal filament, must provide a favorable balance between the kinetics of jet solidification and of jet breakup.
P. Duwez, R. H. Willens and W. Klement in "Continuous Series of Metastable Solid Solution in Ag-Cu Alloys," J. Applied Physics, 31 (1960) 1136-7 disclose a method for the rapid quenching of metal alloys. The process disclosed by Duwez et al. comprises propelling a small liquid droplet, on the order of about 25 mg, by means of a shock wave against the inside surface of a high speed rotating copper annular chill roll or cylinder. The centrifugal force acting on the molten material insures a good thermal contact with the chill surface and the relative motion of the roll and the droplet also helps in spreading the liquid over a larger area. This spreading process leads to a thinner layer of solidified material and therefore a larger over-all thermal transfer rate.
R. Pond, Jr. and R. Maddin in "A Method of Producing Rapidly Solidified Filamentary Castings," Trans. Met. Soc. AIME 245 (1969) 2475-6 expands the concept which Duwez et al. employed to produce metal "splats" to encompass the production of metal filaments. Pond and Maddin disclose an apparatus comprising a small open tube furnace, a pneumatic cage which raised and lowers a graphite ejection mold and a motor-driven chill roll. To operate, the ejection mold is lowered from the furnace into the spinning roll and pressure is applied in the ejection mold which forces a stream of molten alloy through a sapphire orifice onto the inside surface of the spinning roll. The pneumatic cage subsequently pulls the ejection mold out of the chill roll, producing a spiraling specimen on the wall of the chill roll.
Since the ejection mold is pulled out of the cage during production, it is clear that this method can be used to produce only discontinuous lengths. Moreover, although the radial acceleration of the roll induces good thermal contact, it also spreads the stream into a flat filament prior to solidification. Thus, the filaments so produced are flat with blunt edges and have had maximum lengths of up to about seven meters, with thicknesses in the range of 5 to 50 microns and widths of 0.2- 1.5 mm.
There is a need in the art for a simple method for the production of metal filaments, particularly metal filaments having generally round cross sections. More specifically there is a need for continuous lengths of such "round" filaments, particularly those filaments having very fine cross-sections in the range of 0.004 to 0.010 inch. Moreover, there is a need for a simple and direct method for the production of fine diameter filaments of amorphous and metastable alloys, which in many cases can only be obtained by very rapid quenching from the melt, and of metals and metal alloys which are too brittle to be produced in the normal manner.